1. Field of Invention
This invention relates to antennas for transmission and reception of electromagnetic radiation and, in particular, to structures for log-periodic antennas, antennas containing such structures and methods to transmit and detect electromagnetic signals with such antennas.
2. Description of Prior Art
An antenna is a structure (or structures) associated with the transition of electromagnetic energy from propagation in free-space to confined propagation in waveguides, wires, coaxial cables, among other devices (that is, reception), or the reverse process (transmission). The transition from free-space (or “far-field”) propagation to confined propagation is not abrupt but occurs through a “near-field” region in the vicinity of the antenna in which the electromagnetic characteristics are neither those of free-space propagation nor confined propagation. The performance of the antenna as a transmitter or receiver of electromagnetic energy depends upon many factors including the geometric and electromagnetic properties of the antenna as well as the geometric and electromagnetic properties of structures affecting the electromagnetic characteristics of the near-field region. Practical antenna designs need to take into account the effect on antenna performance of structures in the near-field region including transmission lines, electronic detectors (for reception), antenna support members or other nearby objects including, in many cases, the surface of the earth.
Many applications require the detection of very weak electromagnetic signals. In such cases, transmission losses occurring between the antenna and remote electronics can be a serious concern. Thus, antenna designs that permit the location of electronic devices in close proximity to the antenna are desirable for weak signal detection such as commonly arise in the field of radio astronomy, and for transmissions such as deep space communication, or in connection with NASA's deep space network.
Financial support from the SETI Institute, made possible by the Paul G. Allen Foundation, is gratefully acknowledged.
The reciprocity theorem for antennas is a well-known and often-used theorem showing that the performance of an antenna is the same whether it is used in reception or transmission, provided however, that no non-reciprocal devices (such as diodes) are present. For the typical cases considered herein, the reciprocity theorem applies and we describe the performance of antennas either in transmission or reception without distinction.
The performance of many antennas typically depends markedly upon the frequency of the electromagnetic energy transmitted (or received). Such frequency-dependent behavior can be accepted when an antenna is intended to transmit or receive a single frequency or very narrow range of frequencies. However, for other applications it is advantageous that the performance of the antenna be approximately independent of frequency. One example is the search for extraterrestrial intelligence (“SETI”), one aspect of which involves the scanning of relatively large portions of the electromagnetic spectrum for evidence of signals created by extraterrestrial intelligent beings. Clearly, lacking a priori knowledge of the frequency to be analyzed, SETI advantageously employs frequency-independent means for detecting electromagnetic radiation.
According to Rumsey (“Frequency Independent Antennas,” V. H. Rumsey, Academic Press: NY 1966), only an antenna of infinite extent, with a shape specified entirely by angles, can be truly frequency independent. Such idealized shapes are self-similar on all size scales. That is, the geometry of the antenna substructure is the same (except for scale) from infinitely large to infinitely small sizes. In practice, self-similar antenna substructures range from a maximum size to a minimum size with the range of performance (the bandwidth) determined by the largest and smallest substructure dimensions. Among the earliest antennas to show such broadband performance were the planar and conical equiangular spiral designs of Dyson, which meet Rumsey's angular criteria over a limited range of scales (Rumsey supra, pp. 39-53).
A type of antenna which approximates frequency independence has a form which can be specified by two or more angles, a scale factor, and two dimensions. This general form of antenna results from chaining together in electrical contact elements of similar shape in a geometric progression of size to form an antenna consisting of similarly shaped elements or substructures. The dimensions of the smallest and largest elements determine the response bandwidth of the antenna. In transmission, radiation arises from a resonant region of the antenna where adjacent elements behave approximately like a backfire array of switched, half-wave dipoles. Such antennas have electrical and radiation properties which vary periodically with the logarithm of frequency. Some antenna designs permit the scale factor and the unit cell (substructure) shape, defined by angles, to be set to make this frequency variation tolerably small. The resulting “log-periodic” or “LP” antenna is effectively frequency independent over its response bandwidth.
The simple geometry of the self-similar planar switched dipole array is useful for illustrating the general operation of a log-periodic antenna (Rumsey supra, FIG. 5.15 included herein as FIG. 1). Dipoles, 1, are alternately connected to opposite sides of a two-wire transmission line, 2, called a feeder. Signal terminals, 3, are connected to the feeder at the small dipole end. When used in transmission, electromagnetic energy at the operating frequency propagates away from the terminals in the direction of increasing size elements to the “active region” where the dipoles have the correct electric lengths and phases to radiate. Small dipoles near the input are electrically very close (that is, the dipole separation experienced by the electromagnetic wave is small compared to the wavelength) and they generate fields nearly 180 degrees out of phase, which substantially cancel. As the electromagnetic energy travels along the feeder, larger dipoles of increasing separation are encountered. Eventually, a region on the antenna is reached in which the dipoles are phased for backfire radiation (back towards the small dipole end). If the dipoles in this “active” or “resonant” region have electrical lengths of approximately one-half wavelength of the applied signal (the resonance condition) they will generate a beam directed back toward the smaller, non-resonant elements. In a properly designed dipole array antenna, radiation attenuates the input electromagnetic energy or “feeder mode” by more than 20 dB (decibel) as it traverses the active region. If the antenna structure parameters are improperly tuned, a large fraction of the electromagnetic energy will traverse the active region without radiating and be reflected from the wide end of the dipole array. This behavior increases the VSWR (voltage-standing-wave-ratio) of the feeder and enhances the rearward lobe of the radiation pattern, thus increasing the variation of impedance and beamshape over a log-period of frequency. While a nearly unipolar far-field pattern with high gain and linear polarization can be achieved with a planar dipole array, the 3 dB contour of the main lobe is elliptical, making it inefficient for illuminating (or collecting energy from) reflectors which are typically surfaces of rotation.
Among the earliest log-periodic antennas is that of DuHamel and Isabell fabricated from stiff sheet metal and described by Rumsey supra p. 58 and reproduced herein as FIG. 2. This pattern is specified by two angles, a scale factor, and two radial lengths. The antenna can be realized as two separate metal pieces or two slots in an extended metal sheet. If the rays bounding the antenna elements subtend 90 degrees, the geometry is self complementary. In this case, the terminal impedance is 189 ohms and independent of frequency. The radial extent of the antenna and the angle subtended by the flat-top radial teeth determine the minimum frequency of operation. Increasing the radial extent or the angle subtended by the teeth decrease the minimum frequency. The radius of the gap separating the arms to which terminals are attached determines the maximum frequency of operation. From the symmetry of the antenna it is clear that the far-field pattern is bipolar. This pattern is inconvenient for receiving directional signals. While one of the component beams of the bipolar pattern can be terminated with absorber, the maximum directivity of this planar antenna is 9 dB. Also, if the termination is not cooled, the lowest receiver temperature achievable is 150 degrees Kelvin.
If the two arms of the antenna depicted in FIG. 2 are inclined to form a wedge (Rumsey supra, FIG. 5.6, included herein as FIG. 3), the gain of one lobe increases at the expense of the other. When the opening angle of the wedge is reduced to less than approximately 50 degrees, the antenna pattern is effectively unipolar, with the main lobe pointing in the direction of decreasing antenna size.
Variations on this non-planar log-periodic design evolved with straight rather than curved conductor edges. Periodically self-similar patterns composed of symmetric trapezoidal or sawtooth elements played a key role in early theoretical and experimental studies of frequency independent antennas. Rumsey supra FIG. 5.9 (FIG. 4 herein) shows a basic geometry of these structures. The angles, linear dimensions, and scale factors which specify a non-planar log-periodic antenna typically have a critical influence on the behavior of the far-field pattern and impedance over a log-period.
The functioning of a typical non-planar log-periodic antenna can be inferred from near-field measurements for a wire log-periodic antenna analogous to the wire structure depicted in FIG. 4 having wire elements in the approximate shape of triangular, trapezoidal or rectangular teeth. See Rumsey, supra, pp. 66-70. The existence of two modes were shown; a slow wave “transmission line” mode emanating from the antenna vertex and lying substantially within the interior of the wedge, and a radiation mode emanating from an active region of resonant substructure cells. Electric fields for the transmission line mode are polarized roughly linearly between the conductors. Fields for the radiation mode are polarized substantially along the direction of the teeth.
Thus, relative to the wedge geometry of a log-periodic antenna, there are distinct electromagnetic fields lying inside and outside of the wedge. The transmission line mode lies substantially inside the wedge and conducts signals from the narrow end of the wedge where the terminals are located. The radiation mode or radiation response pattern lies substantially outside the wedge. The transmission line and radiation modes are intimately coupled, and changes to the electromagnetic fields inside the antenna wedge result in changes to the radiation mode and, hence, to the performance of the antenna.
In order to connect microwave energy into or out of the terminals, (depending on whether one is transmitting or receiving with the antenna), a transmission line is attached to the antenna terminals. Since transmission lines are conductors, they can disrupt the radiation and transmission modes of the antenna. There are distinct disadvantages to the current transmission line attachments to non-planar log-periodic antennas, among which are the following:                a) Transmission lines attached to the log-periodic antenna terminals typically are routed along the mid-line of one of the antenna arms and out the back (wide end) of the antenna where the lines are attached to an amplifier receiver or transmitter. Attenuation of the signal occurs during transit. This loss is often significant, as high as 1 dB before the signal can be amplified. In addition, the lower the low frequency limit of the antenna, the longer the antenna arm. Hence, a longer transmission line is needed which increases the losses.        b) Receiver/Transmitter electronics are typically separated from the log-periodic antenna structure for, among other reasons, to avoid disruption of the electromagnetic properties of the near-field which typically disrupts the behavior of the antenna. However, to avoid transmission line losses it is useful to integrate an amplifier directly into the antenna. However, any electronic module placed between the antenna arms (inside the wedge geometry) close to the antenna terminals will disrupt the transmission mode feeding the active region of the antenna structure.        
Thus, a need exists in the art for a log-periodic antenna having improved performance and, additionally, for an antenna structure that permits devices to be located in close proximity to the antenna without substantial degradation in performance.